The present invention relates to a command detector of a telephone answering apparatus which enables the telephone subscriber to control the answering apparatus from a remote location in a desired mode.
Telephone answering apparatus developed in the early period of the history of telephone answering art were provided with a tape recorder having a prerecorded message which is played back in response to the reception of a call to advise the calling party of the fact that the called party is absent. In the next stage of development, the answering apparatus was provided with an added feature that allows the user to record messages of callers while he is absent and to playback the recorded messages later. The most recent version of the telephone answering apparatus includes a remote control feature that allows the owner to access to the answering apparatus from a remote location to playback recorded caller's messages or erase unnecessary ones.
The remote control operation involves sending a signal of a particular frequency from a portable device carried by the owner of the telephone answering apparatus into the transmitter of a remote telephone. The command signal is transmitted in the form of a tone burst and includes a remote ready signal having a duration of 2 seconds followed by a start bit of a 0.8-second duration, and mode-select signals to effect rewind and erase. The mode-select code comprises a series of binary "1"s and "0"s of a 50-ms duration of tone burst. The telephone answering apparatus is provided with a command detector which includes plural filters to discriminate between received mode select codes.
It is desirable to provide as many channels of different frequencies to different groups of users to avoid interference between telephone lines. The answering apparatus is thus designed to meet the required sharp frequency selectivity. Although the high selectivity may be achieved by the use of plural mechanical filters of a high Q value connected in successive stages, this adds to the cost of the apparatus. Another shortcoming inherent in the mechanical filter resides in the fact that it has a delay response which makes it difficult to quickly follow the high repetition rate of the tone bursts of the command signal.